1. Field of the Invention
The present invention relates generally to comb filtering apparatus and methods and, in particular, to adaptive comb filters and methods for separating luminance and chrominance signals from a video signal.
2. Description of the Prior Art
Band pass filters have been commonly utilized for separating luminance and chrominance signals of, for example, an NTSC composite color video signal. However, such band pass filters when used alone to effect signal separation are unable to distinguish between chrominance signals and luminance signals falling within the chrominance frequency band, typically 3.58 MHz .+-.500 kHz. This effect, commonly referred to as "cross color", results in the deterioration of the quality of an image produced with the use of chrominance signals separated by band pass filtering.
Comb filters provide improved luminance and chrominance signal separation by utilizing the general vertical correlation of video signals and, in the case of an NTSC or PAL composite color video signal, the phase inversion of the chrominance subcarrier signal which occurs with every new or every other new horizontal line of the video signal, respectively. With reference to FIG. 1, a typical comb filter arrangement for separating luminance signals (indicated as "Y") and chrominance signals (indicated as "C") from an NTSC composite color video signal (indicated as "Y+C") is illustrated therein. The composite color video signal ("Y+C") is applied to an input terminal 101 of the comb filter connected to an input terminal of a delay line (1H) 102 which is operative to provide the composite color video signal "Y+C"delayed by one horizontal line interval at an output terminal thereof. The "Y+C"signal received at the input terminal 101 is also applied to a first input of a subtracting circuit 103 having a second input connected to the output terminal of the delay line 102. Since the luminance signal components of successive horizontal lines are in-phase signals which generally possess vertical correlation, the subtracting circuit 103 serves to attenuate luminance signal components of the delayed composite color video signal "Y+C"as the same is provided at an output terminal thereof.
The output terminal of the subtracting circuit 103 is connected to an input of a band pass filter (BPF) 104 having a pass band corresponding with the frequency range of the chrominance signal "C"and, accordingly, provides a separated chrominance signal component "C"at an output terminal thereof which is connected to an output terminal 105 of the comb filter. The output terminal of the band pass filter 104 is also connected to a first input of a further subtracting circuit 107. A delay line -06 has an input connected to the input terminal 101 to receive the composite color video signal "Y+C"and is operative to delay the "Y+C"signal by an interval corresponding to the delays introduced by the delay line 102 and the band pass filter 104. The delay line 106 provides the delayed composite color video signal "Y+C"to a second input of the subtracting circuit 107. Accordingly, the subtracting circuit 107 serves to attenuate chrominance signal components present in the composite color video signal "Y+C"and thereby provides a separated luminanoe signal component "Y"to an output terminal 108 of the comb filter.
However, vertical correlation of the chrominance signal is not always present. For example, where a change to a complementary color occurs in the chrominance signal in a given scanning line, the phase of the chrominance signal becomes inverted so that vertical correlation thereof with the chrominance signal of the preceding scanning line no longer exists. Not only does this result in the deterioration of the output chrominance signal, but the luminance signal "Y"provided at the output terminal 108 retains portions of the chrominance signal which otherwise would have been suppressed by the subtracting circuit 107, resulting in so-called "dot interference" in the reproduced video image.
On the other hand, where vertical correlation between luminance signals of successive line intervals is lacking, the chrominance signal output by the subtracting circuit 103 possesses a residual component of the luminance signal therein. Accordingly, when this occurs such components may appear at the output of the band pass filter 104, resulting in a cross color effect similar to that encountered in the use of a band pass type signal separation technique of the kind described hereinabove.
In order to overcome the effects resulting from vertical non-correlation, it has been proposed in Japanese Patent application No. 63-301762, assigned to the assignee of the instant application, that vertical correlation of luminance signals in the pass band of the chrominance signal be detected to carry out adaptive comb filtering techniques to avoid such unwanted effects. FIG. 2 illustrates the overall circuit arrangement of the adaptive comb filter disclosed therein.
With reference to FIG. 2, an NTSC composite color video signal "Y+C"is supplied to an input terminal 1 thereof. A first delay line (1H) 2 has an input coupled with the input terminal 1 and is operative to delay the composite color video signal "Y+C"by one horizontal line interval which it then provides at an output terminal thereof. The output terminal of the first delay line 2 is coupled with an input of a second delay line (1H) 3 which is operative to delay the composite color video signal "Y+C"from the output terminal of first delay line 2 by an additional horizontal line interval which it then provides at an output terminal thereof. A first subtracting circuit 4 has two inputs each connected with a respective one of the input terminal 1 and the output terminal of the first delay line 2 and serves to provide a signal at an output terminal thereof representing the difference between the input composite color video signal and a prior version thereof delayed by one horizontal line interval. A second subtracting circuit 5 has two inputs each coupled with a respective one of the output terminals of the first and second delay lines 2 and 3 and serves to provide a signal at an output terminal thereof representing the difference between the composite color video signals respectively delayed by one and two horizontal line intervals with respect to the signal provided at the input terminal 1.
Each of a plurality of band pass filters (BPF) 6, 7, 8, 9 and 10 has a pass band substantially corresponding with the frequency band of the chrominance signal component of the composite color video signal "Y+C". Each of the band pass filters 6-10 has a respective input correspondingly connected with a respective one of the input terminal 1, the output terminal of the first subtracting circuit 4, the output terminal of the first delay line 2, the output terminal of the second subtracting circuit 5 and the output terminal of the second delay line 3. A first adding circuit 11 has a first input connected with an output terminal of the band pass filter 6 and a second input connected with an output terminal of the band pass filter 8. A second adding circuit 12 has a first input connected with the output terminal of the band pass filter 8 and a second input connected with an output terminal of the band pass filter 10.
Each of a pair of absolute value generating circuits (ABS) 13 and 16 is operative to produce absolute value signals at a respective output terminal thereof representing absolute values of signals input thereto. The output terminal of first adding circuit II is connected with the input of absolute value generating circuit 13, while the output terminal of second adding circuit 12 is connected with the input of absolute value generating circuit 16. Each of a pair of coding circuits 17 and 20 is operative to convert signals received at an input thereof to a respective binary code x and y through comparison of the input signals with a predetermined voltage level. The output terminal of absolute value generating circuit 13 is connected to the input of coding circuit 17, while the output terminal of absolute value generating circuit 16 is connected to the input of coding circuit 20. Each of the coding circuits 17 and 20 has an output terminal coupled with a respective input of a logic signal processor 21.
The band pass filters 7 and 9 produce respective output signals X and Y which are produced in the same fashion as the output chrominance signal provided at the output terminal 105 of the comb filter of FIG. 1. The output terminal of the band pass filter 8 is connected with an input of a further band pass filter (BPF) 22 having a pass band similar to that of band pass filters used alone without comb filtering to separate chrominance signals, as first described above. The band pass filter 22 produces an output signal Z corresponding with the output chrominance signal produced by the band pass filtering technique. A switching circuit 23 has a first input terminal 23a connected with the output terminal of the band pass filter 7 to receive the signal X therefrom, a second input terminal 23b connected with the output terminal of the band pass filter 9 to receive the signal Y therefrom and a third input terminal 23c connected with an output terminal of the band pass filter 22 to receive the signal Z therefrom. The switching circuit 23 is operative to connect an output terminal thereof to one of the input terminals 23a-c under the control of the logic signal processor 21.
In addition, the output terminals of the band pass filters 7 and 9 are each connected with an input of a respective absolute value generating circuit (ABS) 14 and 15 which, like circuits 13 and 16, are each operative to produce output signals at an output terminal thereof representing an absolute value of signals input thereto. The output terminals of the absolute value generating circuits 14 and 15 are each connected to an input of a respective one of third and fourth coding circuits 18 and 19 each of which is operative to convert the absolute value signal received thereby to a respective binary code X' and Y' by comparing the absolute value signal with a predetermined voltage level. An output terminal of third coding circuit 18 is connected with an input of the logic signal processor 21 to supply the signal X' thereto, while the fourth coding circuit 19 has an output terminal connected with an input of the logic signal processor 21 to supply the logic signal Y' thereto.
The logic signal processor 21 logically compares the logic signals x, X', Y' and y in accordance with Table I below such that the switching circuit 23 selects a respective one of the signals X, Y and Z in accordance with the signal states indicated in Table I, wherein a dash sign ("--") represents an indefinite state.
TABLE I ______________________________________ selection of input signals x y X' Y' by switching circuit 23 ______________________________________ 0 0 -- -- Y 0 1 -- -- X 1 0 -- -- Y 1 1 -- 0 Y 1 1 0 1 X 1 1 1 1 Z ______________________________________
The output terminal of the switching circuit 23 is connected with an output terminal 24 of the comb filter circuit to provide a chrominance signal component "C" of the input composite color video signal "Y+C" thereto. In order to produce a luminance signal "Y" from the input composite color video signal "Y+C", a third delay line 25 is provided having an input connected with the output terminal of the first delay line 2 and is operative to delay the composite color video signal at the output terminal of the first delay line 2 by an interval corresponding with the signal delay produced by each of the band pass filters 7 and 9. An output terminal of the third delay line 25 is connected with a first input of a third subtracting circuit 26 having a second input connected with the output terminal 24. The subtracting circuit 26 is operative to produce the luminance signal "Y" by subtracting the output chrominance signal "C" supplied at the output terminal 24 from the delayed composite color video signal "Y+C", and provides the thus produced luminance signal "Y" to a luminance signal output terminal 27 of the comb filter circuit.
With reference to FIG. 3, the phases of exemplary output signals provided by the band pass filters 6, 8 and 10 are illustrated therein by waveforms a, b and c, respectively, wherein the waveforms of coincident signals are vertically aligned. Phases of other signals as well as logic levels of coded signals appearing at respective terminals of the FIG. 2 circuit are also illustrated in FIG. 3. In the example of FIG. 3, a phase inversion of the chrominance signal representing a change to a complementary color is represented by a vertical dashed line in each of the waveforms a, b and c. In the example of FIG. 3, the luminance signal is constant.
When the signals represented by the waveforms a, b, and c are supplied as inputs to the first and second adding circuits 11 and 12, they combine subtractively to cancel one another so long as their phases are inverted. However, upon the occurrence of a color change as described above, the phases of these signals on successive lines coincide so that they combine additively as shown for the waveforms "a+b" and "b+c" in FIG. 3, thereupon resulting in a logic "one" output by the respective coding circuits 17 and 20 represented by the signals x and y in FIG. 3 Conversely, the outputs X and Y provided respectively by the band pass filters 7 and 9 from the outputs of the first and second signal subtracting circuits 4 and 5 are eliminated upon the occurrence of a color change.
When this takes place, the circuit of FIG. 1 eliminates the chrominance signal components of the horizontal line interval in which the color change occurs, resulting in dot interference. To avoid this result in the operation of the FIG. 2 adaptive filter circuit, the logic signal processor 21 is operative to control the switch 23 so that it selects the signal X at such times that (x, y)=(0,1) indicating that a color change has occurred between the signals applied to the input terminals of the second adding circuit 12, so that a drop out of the chrominance signal "C" is avoided. It will be seen also from Table I that, upon the occurrence of a color change between the signals input to the first adding circuit 11, resulting in the production of logic signals (x, y)=(1,0), the signal Y is selected instead of the signal X, such that chrominance signal drop out likewise is avoided at such time. Accordingly, a reduction in the level of the chrominance signal "C" due to chrominance signal inversion is thus avoided so that the chrominacne signal "C" produced at the output terminal 24 corresponds with the chrominance signal component in the composite color video signal "Y+C" provided at the output of the delay line 25. Accordingly, a luminance signal "Y" will be provided at the output terminal 27 which is free of dot interference under these circumstances.
Referring now to FIG. 4, the phases of further exemplary output signals from band pass filters 6, 8 and 10 are illustrated therein by waveforms a, b and c respectively, where a chrominance signal first appears on a given horizontal line of the composite color video signal "Y+C", while the level of the luminance signal is constant. Phases and amplitudes of other signals together with logic levels of coded signals appearing at respective terminals of the FIG. 2 circuit are also illustrated in FIG. 4. Where a composite color video signal having these characteristics is applied to the comb filter shown in FIG. 1, a deterioration in the level of the chrominance signal output by the subtracting circuit 103 occurs as shown in the case of the signals X and Y of FIG. 4. The result is a deterioration in the vertical resolution of the image produced with the use of the signals output by the FIG. 1 filter circuit. However, in the case of the adaptive comb filter of FIG. 2, the partially attenuated signals X and Y are rejected thereby such that vertical resolution is improved under these conditions.
With reference to FIG. 5, the phases of still further exemplary output signals from band pass filters 6, 8 and 10 are represented by waveforms a, b and c, respectively, thereof to illustrate luminance signal components within the frequency band of the chrominance signal, such that the phases thereof coincide on successive horizontal scan lines. The corresponding amplitudes and states of signals appearing elsewhere in the circuit of FIG. 2 simultaneously therewith likewise are illustrated in FIG. 5. With reference again to Table I, it will be seen that the presence of such luminance signals at the output of the band pass filter 8 simultaneously with the presence thereof either at the output of band pass filter 6 or 10 results in a logic signal combination (x, y)=(1,1), whereupon the signal Y output by the band pass filter 9 is selected by the switching circuit 23 whenever the signal Y'=0 (to reject the residual luminance signal component concurrently present in the signal X), whereas the signal X is selected by the switching circuit 23 whenever the signal combination (X', Y')=(0,1) (to reject the residual luminance signal component then present in the signal Y). Accordingly, under the circumstances illustrated in FIG. 5, a chrominance signal "C" free of such luminance signal components is produced at the output terminal 24.
Referring now to FIG. 6, outputs from band pass filters 6, 8 and 10 are symbolized therein by the waveforms a, b and c, respectively, which represent the existence of a chrominance signal on only one particular scanning line, while the level of the luminance signal is simultaneously constant. The corresponding amplitudes and states of signals appearing elsewhere in the circuit of FIG. 2 simultaneously therewith likewise ar illustrated in FIG. 6. It will be seen from FIG. 6 that, whether the signal X or the signal Y is selected when this solitary chrominance signal appears at the output of the band pass filter 8, a deterioration in the level thereof is unavoidable. Accordingly, under these circumstances, the chrominance signal Z provided at the output terminal of the band pass filter 22 is selected by the switching circuit 23, as will be seen with reference to Table I under the conditions that x, y, X' and Y' all equal a logic "one" level indicating that neither the luminance signal nor the chrominance signal possesses vertical correlation. Under these circumstances, the signal Z is provided at the output terminal 24 by the switching circuit 23 and is free of the signal deterioration then present in the signals X and Y.
In summary, the first and second adding circuits 11 and 12 and the first and second signal subtracting circuits 4 and 5 serve respectively to produce signals indicating vertical correlation of the luminance signal and of the chrominance signal, so that signals which strongly correlate may be selected to produce the output chrominance signal "C". On the other hand, when the signals do not correlate satisfactorily, the output from the conventional band pass filter circuit is instead selected to produce luminance and chrominance signals of high quality.
With reference now to FIG. 7, wherein elements corresponding to those illustrated in FIG. 2 bear the same reference numerals, a third adding circuit 28 has a first input connected with the output terminal of the band pass filter 9 to receive the signal Y therefrom and a second input connected with the output terminal of the band pass filter 7 to receive the signal X therefrom. The third adding circuit 28 is operative to provide an output signal W in accordance with the following relationship: ##EQU1## Accordingly, the third adding circuit 28 produces a signal W based on the correlation of three horizontal scan lines and corresponding with the output of a so-called 2H comb filter. An output terminal of the third adding circuit 28 is connected to a fourth input 23d of the switching circuit 23.
In the adaptive comb filter of FIG. 7, a modified logic signal processor 100 is operative to control the operation of the switching circuit in accordance with the logic signals x, y, X' and Y' input thereto in accordance with Table II below, the logic signal states are expressed in the same manner as in Table I:
TABLE II ______________________________________ selection of input signals x y X' Y' by switching circuit 23 ______________________________________ 0 0 -- -- W 0 1 -- -- X 1 0 -- -- Y 1 1 -- 0 Y 1 1 0 1 X 1 1 1 1 Z ______________________________________
With reference to Table II, it will be seen that, when signal correlation is present between the signals appearing on the three successive horizontal scanning lines represented by the waveforms a, b and c, the logic signals x and y assume a low or "0" logic level such that the signal W at the output of the third adding circuit 28 is selected by the switching circuit 23. Under the remaining conditions listed in Table II, the selection of the input signals by the switching circuit 23 corresponds to that of the FIG. 2 adaptive comb filter circuit as expressed in Table I.
Referring now to FIG. 8, exemplary chrominance signal waveforms a, b and c appearing at the output terminals of band pass filters 6, 8 and 10, respectively, are illustrated therein where a complementary color change resulting in a phase inversion occurs at successively later intervals differing by one-half of the color subcarrier wavelength from line to line at the output terminals of the band pass filters 6,8 and 10. The corresponding amplitudes and phases of signals appearing elsewhere in the circuit of FIG. 7 therewith simultaneously likewise are illustrated in FIG. 8. Under these circumstance, the signal W is selected except at such times that the signals of two successive lines correspond in phase, whereupon either the signal X or the signal Y is selected to avoid the concurrent deterioration in the level of the signal W. Accordingly, in the adaptive comb filter of FIG. 7, when the signals a, b and c from successive horizontal scan lines exhibit good correlation, the adaptive comb filter acts as a 2H comb filter which provides superior separation of the chrominance signal. When the chrominance signal derived in this manner is subtractively combined with the composite color video signal "Y+C" in the subtracting circuit 26, dot interference can be reduced by 6 dB, thus providing improved image quality.
In the adaptive comb filter circuits of FIGS. 2 and 7, the coding circuits 17-20 compare the input absolute value signals with a pre-determined voltage level. In the event that the level of input signals changes, the coding circuits may be ineffective to properly encode the input signals if the level thereof does not reach the pre-determined voltage level. If the pre-determined voltage level is reduced in an attempt to enable proper encoding of weaker signals, the probability that noise present in the input signal will interfere with proper operation of the adaptive filter circuit is increased.